San Marino?By many accounts, San Marino, one of the world's smallest countries, is the world's oldest country. San Marino, which is completely surrounded by Italy, was founded on September 3 in the year 301 CE. San Marino's constitution is the world's oldest, having been first written in the year 1600. Despite San Marino's founding in the year 301, it wasn't recognized as independent until the year 1631 by the Pope.
Japan?Japan claims an earlier date of founding than San Marino - 660 BCE. According to Japanese history, the country's first emperor, Emperor Jimmu founded Japan in that year. However, it was not until at least the eighth century CE that Japanese culture and Buddhism spread across the islands. Over its long history, Japan had many different types of governments and leaders and while the country celebrates the 660 BCE date as its founding, much of the world sees modern Japan as having come into existence in the nineteenth century of the modern era.
China?The first recorded dynasty in Chinese history existed more than 3500 years ago! The feudal Shang dynasty existed from from the 17th to 11th century BCE. Nonetheless, China celebrates the year 221 BCE as the founding of the modern country. At that point, Qin Shi Huang proclaimed himself the First Emperor of China. In the third century CE, the Han dynasty developed, creating a unified Chinese culture and tradition. In the thirteenth century, the Mongol invasion resulted in a Mongol rule of China, decimating the population and culture. The Republic of China was founded in 1912 and in 1949 China transformed into the People's Republic of China that exists to this day.
Other Contenders for the World's Oldest CountryModern countries such as Egypt, Iraq, Iran, Greece, and India really bear little resemblance to their ancient counterparts and these modern countries do not even consider their "independence" to have taken place in ancient times. All of these countries (except Iran) trace their founding only as far back as the nineteenth century. Iran traces its modern independence to only 1501, with the founding of the Shia Islamic state under the Shah at the time.
Other countries that consider their founding prior to Iran's include France (843 CE), Austria (976 CE), Denmark (10th century), Hungary (1001), Portugal (1143), Mongolia (1206), Thailand (1238), Andorra (1278), Switzerland (1291), Monaco (1419), and Spain (15th century). All of these countries have a long and impressive history, which allows them to maintain their place as some of the oldest nation-states on the planet.
Ultimately, it is difficult to judge which country is the world's oldest due to a variety of complex factors. One might answer that San Marino, Japan, or China is the world's oldest country and still be accurate.